Executing partial-width packed data instructions

ABSTRACT

A method and apparatus are provided for executing scalar packed data instructions. According to one aspect of the invention, a processor includes a plurality of registers, a register renaming unit coupled to the plurality of registers, a decoder coupled to the register renaming unit, and a partial-width execution unit coupled to the decoder. The register renaming unit provides an architectural register file to store packed data operands each of which include a plurality of data elements. The decoder is configured to decode a first and second set of instructions that each specify one or more registers in the architectural register file. Each of the instructions in the first set of instructions specify operations to be performed on all of the data elements stored in the one or more specified registers. In contrast, each of the instructions in the second set of instructions specify operations to be performed on only a subset of the data element stored in the one or more specified registers. The partial-width execution unit is configured to execute operations specified by either of the first or the second set of instructions.

FIELD OF THE INVENTION

The invention relates generally to the field of computer systems. Moreparticularly, the invention relates to a method and apparatus forefficiently executing partial-width packed data instructions, such asscalar packed data instructions, by a processor that makes use of SIMDtechnology, for example.

BACKGROUND OF THE INVENTION

Multimedia applications such as 2D/3D graphics, image processing, videocompression/decompression, voice recognition algorithms and audiomanipulation, often require the same operation to be performed on alarge number of data items (referred to as “data parallelism”). Eachtype of multimedia application typically implements one or morealgorithms requiring a number of floating point or integer operations,such as ADD or MULTIPLY (hereafter MUL). By providing macro instructionswhose execution causes a processor to perform the same operation onmultiple data items in parallel, Single Instruction Multiple Data (SIMD)technology, such as that employed by the Pentium® processor architectureand the MMx™ instruction set, has enabled a significant improvement inmultimedia application performance (Pentium® and MMx™ are registeredtrademarks or trademarks of Intel Corporation of Santa Clara, Calif.).

SIMD technology is especially suited to systems that provide packed dataformats. A packed data format is one in which the bits in a register arelogically divided into a number of fixed-sized data elements, each ofwhich represents a separate value. For example, a 64-bit register may bebroken into four 16-bit elements, each of which represents a separate16-bit value. Packed data instructions may then separately manipulateeach element in these packed data types in parallel.

Referring to FIG. 1, an exemplary packed data instruction isillustrated. In this example, a packed ADD instruction (e.g., a SIMDADD) adds corresponding data elements of a first packed data operand, X,and a second packed data operand, Y, to produce a packed data result, Z,i.e., X₀+Y₀=Z₀, X₁+Y₁=Z₁, X₂+Y₂=Z₂, and X₃+Y₃=Z₃. Packing many dataelements within one register or memory location and employing parallelhardware execution allows SIMD architectures to perform multipleoperations at a time, resulting in significant performance improvement.For instance, in this example, four individual results may be obtainedin the time previously required to obtain a single result.

While the advantages achieved by SIMD architectures are evident, thereremain situations in which it is desirable to return individual resultsfor only a subset of the packed data elements.

SUMMARY OF THE INVENTION

A method and apparatus are described for executing partial-width packeddata instructions. According to one aspect of the invention, a processorincludes a plurality of registers, a register renaming unit coupled tothe plurality of registers, a decoder coupled to the register renamingunit, and a partial-width execution unit coupled to the decoder. Theregister renaming unit provides an architectural register file to storepacked data operands each of which include a plurality of data elements.The decoder is configured to decode a first and second set ofinstructions that each specify one or more registers in thearchitectural register file. Each of the instructions in the first setof instructions specify operations to be performed on all of the dataelements stored in the one or more specified registers. In contrast,each of the instructions in the second set of instructions specifyoperations to be performed on only a subset of the data element storedin the one or more specified registers. The partial-width execution unitis configured to execute operations specified by either of the first orthe second set of instructions.

Other features and advantages of the invention will be apparent from theaccompanying drawings and from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by way of example and not by way oflimitation with reference to the figures of the accompanying drawings inwhich like reference numerals refer to similar elements and in which:

FIG. 1 illustrates a packed ADD instruction adding togethercorresponding data elements from a first packed data operand and asecond packed data operand.

FIG. 2A is a simplified block diagram illustrating an exemplary computersystem according to one embodiment of the invention.

FIG. 2B is a simplified block diagram illustrating exemplary sets oflogical registers according to one embodiment of the invention.

FIG. 2C is a simplified block diagram illustrating exemplary sets oflogical registers according to another embodiment of the invention.

FIG. 3 is a flow diagram illustrating instruction execution according toone embodiment of the invention.

FIG. 4 conceptually illustrates the result of executing a partial-widthpacked data instruction according to various embodiments of theinvention.

FIG. 5A conceptually illustrates circuitry for executing full-widthpacked data instructions and partial-width packed data instructionsaccording to one embodiment of the invention.

FIG. 5B conceptually illustrates circuitry for executing full-widthpacked data and partial-width packed data instructions according toanother embodiment of the invention.

FIG. 5C conceptually illustrates circuitry for executing full-widthpacked data and partial-width packed data instructions according to yetanother embodiment of the invention.

FIG. 6 illustrates an ADD execution unit and a MUL execution unitcapable of operating as four separate ADD execution units and fourseparate MUL execution units, respectively, according to an exemplaryprocessor implementation of SIMD.

FIGS. 7A-7B conceptually illustrate a full-width packed data operationand a partial-width packed data operation being performed in a“staggered” manner, respectively.

FIG. 8A conceptually illustrates circuitry within a processor thataccesses full width operands from logical registers while performingoperations on half of the width of the operands at a time.

FIG. 8B is a timing chart that further illustrates the circuitry of FIG.8A.

FIG. 9 conceptually illustrates one embodiment of an out-of-orderpipeline to perform operations on operands in a “staggered” manner byconverting a macro instruction into a plurality of micro instructionsthat each processes a portion of the full width of the operands.

FIG. 10 is a timing chart that further illustrates the embodimentdescribed in FIG. 9.

FIG. 11 is a block diagram illustrating decoding logic that may beemployed to accomplish the decoding processing according to oneembodiment of the invention.

DETAILED DESCRIPTION

A method and apparatus are described for performing partial-width packeddata instructions. Herein the term “full-width packed data instruction”is meant to refer to a packed data instruction (e.g., a SIMDinstruction) that operates upon all of the data elements of one or morepacked data operands. In contrast, the term “partial-width packed datainstruction” is meant to broadly refer to a packed data instruction thatis designed to operate upon only a subset of the data elements of one ormore packed data operands and return a packed data result (to a packeddata register file, for example). For instance, a scalar SIMDinstruction may require only a result of an operation between the leastsignificant pair of packed data operands. In this example, the remainingdata elements of the packed data result are disregarded as they are ofno consequence to the scalar SIMD instruction (e.g., the remaining dataelements are don't cares). According to the various embodiments of theinvention, execution units may be configured in such a way toefficiently accommodate both full-width packed data instructions (e.g.,SIMD instructions) and a set of partial-width packed data instructions(e.g., scalar SIMD instructions).

In the following detailed description, for purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the invention. It will be apparent, however, to one ofordinary skill in the art that these specific details need not be usedto practice the invention. In other instances, well-known devices,structures, interfaces, and processes have not been shown or are shownin block diagram form.

justification of Partial-Width Packed Data Instructions

Considering the amount of software that has been written for scalararchitectures (e.g., single instruction single data (SISD)architectures) employing scalar operations on single precision floatingpoint data, double precision floating point data, and integer data, itis desirable to provide developers with the option of porting theirsoftware to architectures that support packed data instructions, such asSIMD architectures, without having to rewrite their software and/orlearn new instructions. By providing partial-width packed datainstructions, a simple translation can transform old scalar code intoscalar packed data code. For example, it would be very easy for acompiler to produce scalar SIMD instructions from scalar code. Then, asdevelopers recognize portions of their software that can be optimizedusing SIMD instructions, they may gradually take advantage of the packeddata instructions. Of course, computer systems employing SIMD technologyare likely to also remain backwards compatible by supporting SISDinstructions as well. However, the many recent architecturalimprovements and other factors discussed herein make it advantageous fordevelopers to transition to and exploit SIMD technology, even if onlyscalar SIMD instructions are employed at first.

Another justification for providing partial-width packed datainstructions is the many benefits which may be achieved by operating ononly a subset of a full-width operand, including reduced powerconsumption, increased speed, a clean exception model, and increasedstorage. As illustrated below, based on an indication provided with thepartial-width packed data instruction, power savings may be achieved byselectively shutting down those of the hardware units that areunnecessary for performing the current operation.

Another situation in which it is undesirable to force a packed datainstruction to return individual results for each pair of data elementsincludes arithmetic operations in an environment providing partial-widthhardware. Due to cost and/or die limitations, it is common not toprovide full support for certain arithmetic operations, such as dividethe By its nature, the divide operation is very long, even whenfull-width hardware (e.g., a one-to-one correspondence between executionunits and data elements) is implemented,. Therefore, in an environmentthat supports only full-width packed data operations while providingpartial-width hardware, the latency becomes even longer. As will beillustrated further below, a partial-width packed data operation, suchas a partial-width packed data divide operation, may selectively allowcertain portions of its operands to bypass the divide hardware. In thismanner, no performance penalty is incurred by operating upon only asubset of the data elements in the packed data operands.

Additionally, exceptions raised in connection with extraneous dataelements may cause confusion to the developer and/or incompatibilitybetween SISD and SIMD machines. Therefore, it is advantageous to reportexceptions for only those data elements upon which the instruction ismeant to operate. Partial-width packed data instruction support allows apredictable exception model to be achieved by limiting the triggering ofexceptional conditions to those raised in connection with the dataelements being operated upon or in which exceptions produced byextraneous data elements would be likely to cause confusion orincompatibility between SISD and SIMD machines.

Finally, in embodiments where portions of destination packed dataoperand is not corrupted as a result of performing a partial-widthpacked data operation, partial-width packed data instructionseffectively provide extra register space for storing data. For instance,if the lower portion of the packed data operand is being operated upon,data may be stored in the upper portion and vice versa.

An Exemplary Computer System

FIG. 2A is a simplified block diagram illustrating an exemplary computersystem according to one embodiment of the invention. In the embodimentdepicted, computer system 200 includes a processor 205, a storage device210, and a bus 215. The processor 205 is coupled to the storage device210 by the bus 215. In addition, a number of user input/output devices,such as a keyboard 220 and a display 225 are also coupled to bus 215.The computer system 200 may also be coupled to a network 230 via bus215. The processor 205 represents a central processing unit of any typeof architecture, such as a CISC, RISC, VLIW, or hybrid architecture. Inaddition, the processor 205 may be implemented on one or more chips. Thestorage device 210 represents one or more mechanisms for storing data.For example, the storage device 210 may include read only memory (ROM),random access memory (RAM), magnetic disk storage mediums, opticalstorage mediums, flash memory devices, and/or other machine-readablemediums. The bus 215 represents one or more buses (e.g., AGP, PCI, ISA,X-Bus, EISA, VESA, etc.) and bridges (also termed as bus controllers).While this embodiment is described in relation to a single processorcomputer system, it is appreciated that the invention may be implementedin a multi-processor computer system. In addition while the presentembodiment is described in relation to a 32-bit and a 64-bit computersystem, the invention is not limited to such computer systems.

FIG. 2A additionally illustrates that the processor 205 includes aninstruction set unit 260. Of course, processor 205 contains additionalcircuitry; however, such additional circuitry is not necessary tounderstanding the invention. An any rate, the instruction set unit 260includes the hardware and/or firmware to decode and execute one or moreinstruction sets. In the embodiment depicted, the instruction set unit260 includes a decode/execution unit 275. The decode unit decodesinstructions received by processor 205 into one or more microinstructions. The execution unit performs appropriate operations inresponse to the micro instructions received from the decode unit. Thedecode unit may be implemented using a number of different mechanisms(e.g., a look-up table, a hardware implementation, a PLA, etc.).

In the present example, the decode/execution unit 275 is showncontaining an instruction set 280 that includes both full-width packeddata instructions and partial-width packed data instructions. Thesepacked data instructions, when executed, may cause the processor 205 toperform full-/partial-width packed floating point operations and/orfull-/partial-width packed integer operations. In addition to the packeddata instructions, the instruction set 280 may include otherinstructions found in existing micro processors. By way of example, inone embodiment the processor 205 supports an instruction set which iscompatible with Intel 32-bit architecture (IA-32) and/or Intel 64-bitarchitecture (IA-64).

A memory unit 285 is also included in the instruction set unit 260. Thememory unit 285 may include one or more sets of architectural registers(also referred to as logical registers) utilized by the processor 205for storing information including floating point data and packedfloating point data. Additionally, other logical registers may beincluded for storing integer data, packed integer data, and variouscontrol data, such as a top of stack indication) and the like. The termsarchitectural register and logical register are used herein to refer tothe concept of the manner in which instructions specify a storage areathat contains a single operand. Thus, a logical register may beimplemented in hardware using any number of well known techniques,including a dedicated physical register, one or more dynamicallyallocated physical registers using a register renaming mechanism(described in further detail below), etc. In any event, a logicalregister represents the smallest unit of storage addressable by a packeddata instruction.

In the embodiment depicted, the storage device 210 has stored therein anoperating system 235 and a packed data routine 240 for execution by thecomputer system 200. The packed data routine 240 is a sequence ofinstructions that may include one or more packed data instructions, suchas scalar SIMD instructions or SIMD instructions. As discussed furtherbelow, there are situations, including speed, power consumption andexception handling, where it is desirable to perform an operation on (orreturn individual results for) only a subset of data elements in apacked data operand or a pair of packed data operands. Therefore, it isadvantageous for processor 205 to be able to differentiate betweenfull-width packed data instructions and partial-width packed datainstructions and to execute them accordingly.

FIG. 2B is a simplified block diagram illustrating exemplary sets oflogical registers according to one embodiment of the invention. In thisexample, the memory unit 285 includes a plurality of scalar floatingpoint registers 291 (a scalar register file) and a plurality of packedfloating point registers 292 (a packed data register file). The scalarfloating point registers 291 (e.g., registers R₀-R₇) may be implementedas a stack referenced register file when floating point instructions areexecuted so as to be compatible with existing software written for theIntel Architecture. In alternative embodiments, however, the registers291 may be treated as a flat register file. In the embodiment depicted,each of the packed floating point registers (e.g., XMM₀-XMM₇) areimplemented as a single 128-bit logical register. It is appreciated,however, wider or narrower registers may be employed to conform to animplementation that uses more or less data elements or larger or smallerdata elements. Additionally, more or less packed floating pointregisters 292 may be provided. Similar to the scalar floating pointregisters 291, the packed floating point registers 292 may beimplemented as either a stack referenced register file or a flatregister file when packed floating point instructions are executed.

FIG. 2C is a simplified block diagram illustrating exemplary sets oflogical registers according to another embodiment of the invention. Inthis example, the memory unit 285, again, includes a plurality of scalarfloating point registers 291 (a scalar register file) and a plurality ofpacked floating point registers 292 (a packed data register file).However, in the embodiment depicted, each of the packed floating pointregisters (e.g., XMM₀-XMM₇) are implemented as a corresponding pairs ofhigh 293 and low registers 294. As will be discussed further below, itis advantageous for purposes of instruction decoding to organize thelogical register address space for the packed floating point registers292 such that the high and low register pairs differ by a single bit.For example, the high and low portions of XMM₀-XMM₇ may bedifferentiated by the MSB. Preferably, each of the packed floating pointregisters 291 are wide enough to accommodate four 32-bit singleprecision floating point data elements. As above, however, wider ornarrower registers may be employed to conform to an implementation thatuses more or less data elements or larger or smaller data elements.Additionally, while the logical packed floating point registers 292 inthis example each comprise corresponding pairs of 64-bit registers, inalternative embodiments each packed floating point register may compriseany number of registers.

Instruction Execution Overview

Having described an exemplary computer system in which one embodiment ofthe invention may be implemented, instruction execution will now bedescribed.

FIG. 3 is a flow diagram illustrating instruction execution according toone embodiment of the invention. At step 310, an instruction is receivedby the processor 205. At step 320, based on the type of instruction,partial-width packed data instruction (e.g., scalar SIMD instruction) orfull-width packed data instruction (e.g., SIMD instruction), processingcontinues with step 330 or step 340. Typically, in the decode unit thetype of instruction is determined based on information contained withinthe instruction. For example, information may be included in a prefix orsuffix that is appended to an opcode or provided via an immediate valueto indicate whether the corresponding operation is to be performed onall or a subset of the data elements of the packed data operand(s). Inthis manner, the same opcodes may be used for both full-width packeddata operations and partial-width packed data operations. Alternatively,one set of opcodes may be used for partial-width packed data operationsand a different set of opcodes may be used for full-width packed dataoperations.

In any event, if the instruction is a conventional full-width packeddata instruction, then at step 330, a packed data result is determinedby performing the operation specified by the instruction on each of thedata elements in the operand(s). However, if the instruction is apartial-width packed data instruction, then at step 340, a first portionof the result is determined by performing the operation specified by theinstruction on a subset of the data elements and the remainder of theresult is set to one or more predetermined values. In one embodiment,the predetermined value is the value of the corresponding data elementin one of the operands. That is, data elements may be “passed through”from data elements of one of the operands to corresponding data elementsin the packed data result. In another embodiment, the data elements inthe remaining portion of the result are all cleared (zeroed). Exemplarylogic for performing the passing through of data elements from one ofthe operands to the result and exemplary logic for clearing dataelements in the result are described below.

FIG. 4 conceptually illustrates the result of executing a partial-widthpacked data instruction according to various embodiments of theinvention. In this example, an operation is performed on data elementsof two logical source registers 410 and 420 by an execution unit 440.The execution unit 440 includes circuitry and logic for performing theoperation specified by the instruction. In addition, the execution unit440 may include selection circuitry that allows the execution unit 440to operate in a partial-width packed data mode or a full-width packeddata mode. For instance, the execution unit 440 may include pass throughcircuitry to pass data elements from one of the logical source registers410, 420 to the logical destination register 430, or clearing circuitryto clear one or more data elements of the logical destination register430, etc. Various other techniques may also be employed to affect theresult of the operation, including forcing one of the inputs to theoperation to a predetermined value, such as a value that would cause theoperation to perform its identity function or a value that may passthrough arithmetic operations without signaling an exception (e.g., aquiet not-a-number (QNaN)).

In the example illustrated, only the result (Z₀) of the operation on thefirst pair of data elements (X₀ and Y₀) is stored in the logicaldestination register 430. Assuming the execution unit 440 includes passthrough logic, the remaining data elements of the logical destinationregister 430 are set to values from corresponding data elements oflogical source register 410 (i.e., X₃, X₂, and X₁). While the logicaldestination register 430 is shown as a separate logical register, it isimportant to note that it may concurrently serve as one of the logicalsource registers 410, 420. Therefore, it should be appreciated thatsetting data elements of the logical destination register 430 to valuesfrom one of the logical source registers 410, 420 in this context mayinclude doing nothing at all. For example, in the case that logicalsource register 410 is both a logical source and destination register,various embodiments may take advantage of this and simply not touch oneor more of the data elements which are to be passed through.

Alternatively, the execution unit 440 may include clearing logic. Thus,rather than passing through values from one of the logical sourceregisters to the logical destination register 430, those of the dataelements in the result that are unnecessary are cleared. Again, in thisexample, only the result (Z₀) of the operation on the first pair of dataelements (X₀ and Y₀) is stored in the logical destination register 430.The remaining data elements of the logical destination register 430 are“cleared” (e.g., set to zero, or any other predetermined value for thatmatter).

Full-Width Hardware

FIGS. 5A-5C conceptually illustrate execution units 540, 560 and 580,respectively, which may execute both full-width packed data andpartial-width packed data instructions. The selection logic included inthe execution units of FIGS. 5A and 5C represent exemplary pass throughlogic, while the selection logic of FIG. 5B is representative ofclearing logic that may be employed. In the embodiments depicted, theexecution units 540, 560, and 580 each include appropriate logic,circuitry and/or firmware for concurrently performing an operation 570,571, and 572 on the full-width of the operands (X and Y).

Referring now to FIG. 5A, the execution unit 540 includes selectionlogic (e.g., multiplexers (MUXes) 555-557) for selecting between a valueproduced by the operation 570 and a value from a corresponding dataelement of one of the operands. The MUXes 555-557 may be controlled, forexample, by a signal that indicates whether the operation currentlybeing executed is a full-width packed data operation or a partial-widthpacked data operation. In alternative embodiments, additionalflexibility may be achieved by including an additional MUX for dataelement 0 and/or independently controlling each MUX. Various means ofproviding MUX control are possible. According to one embodiment, suchcontrol may originate or be derived from the instruction itself or maybe provided via immediate values. For example, a 4-bit immediate valueassociated with the instruction may be used to allow the MUXes 555-557to be controlled directly by software. Those MUXes corresponding to aone in the immediate value may be directed to select the result of theoperation while those corresponding to a zero may be caused to selectthe pass through data. Of course, more or less resolution may beachieved in various implementations by employing more or less bits torepresent the immediate value.

Turning now to FIG. 5B, the execution unit 540 includes selection logic(e.g., MUXes 565-567) for selecting between a value produced by anoperation 571 and a predetermined value (e.g., zero). As above, theMUXes 565-567 may be under common control or independently controlled.

The pass through logic of FIG. 5C (e.g., MUXes 575-576) selects betweena data element of one of the operands and an identity function value590. The identity function value 590 is generally chosen such that theresult of performing the operation 572 between the identity functionvalue 590 and the data element is the value of the data element. Forexample, if the operation 572 was a multiply operation, then theidentity function value 590 would be 1. Similarly, if the operation 572was an add operation, the identity function value 590 would be 0. Inthis manner, the value of a data element can be selectively passedthrough to the logical destination register 430 by causing thecorresponding MUX 575-577 to output the identity function value 590.

In the embodiments described above, the circuitry was hardwired suchthat the partial-width operation was performed on the least significantdata element portion. It is appreciated that the operation may beperformed on a different data element portions than illustrated. Also,as described above, the data elements to be operated upon may be made tobe software configurable by coupling all of the operations to a MUX orthe like, rather than simply a subset of the operations as depicted inFIGS. 5A-5C. Further, while pass through and clearing logic aredescribed as two options for treating resulting data elementscorresponding to operations that are to be disregarded, alternativeembodiments may employ other techniques. For example, a QNaN may beinput as one of the operands to an operation whose result is to bedisregarded. In this manner, arithmetic operations compliant with theIEEE 754 standard, IEEE std. 754-1985, published Mar. 21, 1985, willpropagate a NaN through to the result without triggering an arithmeticexception.

While no apparent speed up would be achieved in the embodimentsdescribed above since the full-width of the operands can be processed inparallel, it should be appreciated that power consumption can be reducedby shutting down those of the operations whose results will bedisregarded. Thus, significant power savings may be achieved.Additionally, with the use of QNaNs and/or identity function values apredictable exception model may be maintained by preventing exceptionsfrom being triggered by data elements that are not part of thepartial-width packed data operation. Therefore, reported exceptions arelimited to those raised in connection with the data element(s) uponwhich the partial-width packed data operation purports to operate.

FIG. 6 illustrates a current processor implementation of an arithmeticlogic unit (ALU) that can be used to execute full-width packed datainstructions. The ALU of FIG. 6 includes the circuitry necessary toperform operations on the full width of the operands (i.e., all of thedata elements). FIG. 6 also shows that the ALU may contain one or moredifferent types of execution units. In this example, the ALU includestwo different types of execution units for respectively performingdifferent types of operations (e.g., certain ALUs use separate units forperforming ADD and MUL operations). The ADD execution unit and the MULexecution unit are respectively capable of operating as four separateADD execution units and four separate MUL execution units.Alternatively, the ALU may contain one or more Multiply Accumulate (MAC)units, each capable of performing more than a single type of operation.While the following examples assume the use of ADD and MUL executionunits and floating point operations, it is appreciated other executionunits such as MAC and/or integer operations may also be used. Further,it may be preferable to employ a partial-width implementation (e.g., animplementation with less than a one-to-one correspondence betweenexecution units and data elements) and additional logic to coordinatereuse of the execution units as described below.

Partial-Width Hardware and “Staggered Execution”

FIGS. 7A-7B conceptually illustrate a full-width packed data operationand a partial-width packed data operation being performed in a“staggered” manner, respectively. “Staggered execution” in the contextof this embodiment refers to the process of dividing each of aninstruction's operands into separate segments and sequentiallyprocessing each segment using the same hardware. The segments aresequentially processed by introducing a delay into the processing of thesubsequent segments. As illustrated in FIGS. 7A-7B, in both cases, thepacked data operands are divided into a “high order segment” (dataelements 3 and 2) and a “low order segment” (data elements 1 and 0). Inthe example of FIG. 7A, the low order segment is processed while thehigh order segment is delayed. Subsequently, the high order segment isprocessed and the full-width result is obtained. In the example of FIG.7B, the low order segment is processed, while whether the high orderdata segment is processed depends on the implementation. For example,the high order data segment may not need to be processed if thecorresponding result is to be zeroed. Additionally, it is appreciatedthat if the high order data segment is not processed, then both the highand low order data segments may be operated upon at the same time.Similarly, in a full-width implementation (e.g., an implementation witha one-to-one correspondence between execution units and data elements)the high and low order data segments may be processed concurrently or asshown in FIG. 7A.

Additionally, although the following embodiments are described as havingonly ADD and MUL execution units, other types of execution units such asMAC units may also be used.

While there are a number of different ways in which the staggeredexecution of instructions can be achieved, the following sectionsdescribe two exemplary embodiments to illustrate this aspect of theinvention. In particular, both of the described exemplary embodimentsreceive the same macro instructions specifying logical registerscontaining 128 bit operands.

In the first exemplary embodiment, each macro instruction specifyinglogical registers containing 128 bit operands causes the full-width ofthe operands to be accessed from the physical registers. Subsequent toaccessing the full-width operands from the registers, the operands aredivided into the low and high order segments (e.g., using latches andmultiplexers) and sequentially executed using the same hardware. Theresulting half-width results are collected and simultaneously written toa single logical register.

In contrast, in the second exemplary embodiment each macro instructionspecifying logical registers containing 128 bit operands is divided intoat least two micro instructions that each operate on only half of theoperands. Thus, the operands are divided into a high and low ordersegment and each micro instruction separately causes only half of theoperands to be accessed from the registers. This type of a division ispossible in a SIMD architecture because each of the operands isindependent from the other. While implementations of the secondembodiment can execute the micro instructions in any order (either an inorder or an out of order execution model), the micro instructionsrespectively cause the operation specified by the macro instruction tobe independently or separately performed on the low and high ordersegments of the operands. In addition, each micro instruction causeshalf of the resulting operand to be written into the single destinationlogical register specified by the macro instruction.

While embodiments are described in which 128 bit operands are dividedinto two segments, alternative embodiments could use larger or smalleroperands and/or divide those operands into more than two segments. Inaddition, while two exemplary embodiments are described for performingstaggered execution, alternative embodiments could use other techniques.

First Exemplary Embodiment Employing “Staggered Execution”

FIG. 8A conceptually illustrates circuitry within a processor accordingto a first embodiment that accesses full width operands from the logicalregisters but that performs operations on half of the width of theoperands at a time. This embodiment assumes that the processor executionengine is capable of processing one instruction per clock cycle. By wayof example, assume the following sequence of instructions is executed:ADD X, Y; MUL A, B. At time T, 128-bits of X and 128-bits of Y are eachretrieved from their respective physical registers via ports 1 and 2.The lower order data segments, namely the lower 64 bits, of both X and Yare passed into multiplexers 802 and 804 and then on to the executionunits for processing. The higher order data segments, the higher 64 bitsof X and Y are held in delay elements M1 and M2. At time T+1, the higherorder data segments of X and Y are read from delay elements M1 and M2and passed into multiplexers 802 and 804 and then on to the executionunits for processing. In general, the delay mechanism of storing thehigher order data segments in delay elements M1 and M2 allows N-bit(N=64 in this example) hardware to process 2N-bits of data. The loworder results from the execution unit are then held in delay element M3until the high order results are ready. The results of both processingsteps are then written back to register file 800 via port 3. Recall thatin the case of a partial-width packed data operation one or more dataelements of the low or high order results may be forced to apredetermined value (e.g., zero, the value of a corresponding dataelement in one of X or Y, etc.) rather than the output of the ADD or MULoperation.

Continuing with the present example, at time T+1, the MUL instructionmay also have been started. Thus, at time T+1, 128-bits of A and B mayeach have been retrieved from their respective registers via ports 1 and2. The lower order data segments, namely the lower 64-bits, of both Aand B may be passed into multiplexers 806 and 808. After the higherorder bits of X and Y are removed from delay elements M1 and M2 andpassed into multiplexers 806 and 808, the higher order bits of A and Bmay be held in storage in delay elements M1 and M2. The results of bothprocessing steps is written back to register file 800 via port 3.

Thus, according to an embodiment of the invention, execution units areprovided that contain only half the hardware (e.g. two single precisionADD execution units and two single precision MUL execution units),instead of the execution units required to process the full width of theoperands in parallel as found in a current processor. This embodimenttakes advantage of statistical analysis showing that multimediaapplications utilize approximately fifty percent ADD instructions andfifty percent MUL instructions. Based on these statistics, thisembodiment assumes that multimedia instructions generally follow thefollowing pattern: ADD, MUL, ADD, MUL, etc. By utilizing the ADD and MULexecution units in the manner described above, the present embodimentprovides for an optimized use of the execution units, thus enablingcomparable performance to the current processor, but at a lower cost.

FIG. 8B is a timing chart that further illustrates the circuitry of FIG.8A. More specifically, as illustrated in FIG. 8B, when instruction “ADDX, Y” is issued at time T, the two ADD execution units first performADDs on the lower order data segments or the lower two packed dataelements of FIG. 1, namely X₀Y₀ and X₁Y₁. At time T+1, the ADD operationis performed on the remaining two data elements from the operands, bythe same execution units, and the subsequent two data elements of thehigher order data segment are added, namely X₂Y₂ and X₃Y₃. While theabove embodiment is described with reference to ADD and MUL operationsusing two execution units, alternate embodiments may use any number ofexecution units and/or execute any number of different operations in astaggered manner.

According to this embodiment, 64-bit hardware may be used to process128-bit data. A 128-bit register may be broken into four 32-bitelements, each of which represents a separate 32-bit value. At time T,the two ADD execution units perform ADDs first on the two lower 32-bitvalues, followed by an ADD on the higher 32-bit values at time T+1. Inthe case of a MUL operation, the MUL execution units behave in the samemanner. This ability to use currently available 64-bit hardware toprocess 128-bit data represents a significant cost advantage to hardwaremanufacturers.

As described above, the ADD and MUL execution units according to thepresent embodiment are reused to reexecute a second ADD or MUL operationat a subsequent clock cycle. Of course, in the case of a partial-widthpacked data instruction, the execution units are reused but theoperation is not necessarily reexecuted since power to the executionunit may be selectively shut down. At any rate, as described earlier, inorder for this re-using or “staggered execution” to perform efficiently,this embodiment takes advantage of the statistical behavior ofmultimedia applications.

If a second ADD instruction follows a first ADD instruction, the secondADD may be delayed by a scheduling unit to allow the ADD execution unitsto complete the first ADD instruction, or more specifically on thehigher order data segment of the first ADD instruction. The second ADDinstruction may then begin executing. Alternatively, in an out-of-orderprocessor, the scheduling unit may determine that a MUL instructionfurther down the instruction stream may be performed out-of-order. Ifso, the scheduling unit may inform the MUL execution units to beginprocessing the MUL instruction. If no MUL instructions are available forprocessing at time T+1, the scheduler will not issue an instructionfollowing the first ADD instruction, thus allowing the ADD executionunits time to complete the first ADD instruction before beginning thesecond ADD instruction.

Yet another embodiment of the invention allows for back-to-back ADD orMUL instructions to be issued by executing the instructions on the sameexecution units on half clock cycles instead of full clock cycles.Executing an instruction on the half clock cycle effectively “doublepumps” the hardware, i.e. makes the hardware twice as fast. In thismanner, the ADD or MUL execution units may be available during eachclock cycle to process a new instruction. Double pumped hardware wouldallow for the hardware units to execute twice as efficiently as singlepumped hardware that executes only on the full clock cycle. Doublepumped hardware requires significantly more hardware, however, toeffectively process the instruction on the half clock cycle.

It will be appreciated that modifications and variations of theinvention are covered by the above teachings and within the purview ofthe appended claims without departing from the spirit and intended scopeof the invention. For example, although only two execution units aredescribed above, any number of logic units may be provided.

Second Exemplary Embodiment Employing “Staggered Execution”

According to an alternate embodiment of the invention, the staggeredexecution of a full width operand is achieved by converting a full widthmacro instruction into at least two micro instructions that each operateon only half of the operands. As will be described further below, whenthe macro instruction specifies a partial-width packed data operation,better performance can be achieved by eliminating micro instructionsthat are not necessary for the determination of the partial-widthresult. In this manner, processor resource constraints are reduced andthe processor is not unnecessarily occupied with inconsequential microinstructions. Although the description below is written according to aparticular register renaming method, it will be appreciated that otherregister renaming mechanisms may also be utilized consistent with theinvention. The register renaming method as described below assumes theuse of a Register Alias Table (RAT), a Reorder Buffer (ROB) and aretirement buffer, as described in detail in U.S. Pat. No. 5,446,912.Alternate register renaming methods such as that described in U.S. Pat.No. 5,197,132 may also be implemented.

FIG. 9 conceptually illustrates one embodiment of a pipeline to performoperations on operands in a “staggered” manner by converting a macroinstruction into a plurality of micro instructions that each processes aportion of the full width of the operands. It should be noted thatvarious other stages of the pipeline, e.g. a prefetch stage, have notbeen shown in detail in order not to unnecessarily obscure theinvention. As illustrated, at the decode stage of the pipeline, a fullwidth macro instruction is received, specifying logical sourceregisters, each storing a full width operand (e.g. 128-bit). By way ofexample, the described operands are 128-bit packed floating point dataoperands. In this example, the processor supports Y logical registersfor storing packed floating point data. The macro instruction isconverted into micro instructions, namely a “high order operation” and a“low order operation,” that each cause the operation of the macroinstruction to be performed on half the width of the operands (e.g., 64bits).

The two half width micro instructions then move into a register renamingstage of the pipeline. The register renaming stage includes a variety ofregister maps and reorder buffers. The logical source registers of eachmicro instruction are pointers to specific register entries in aregister mapping table (e.g. a RAT). The entries in the register mappingtable in turn point to the location of the physical source location inan ROB or in a retirement register. According to one embodiment, inorder to accommodate the half width high and low order operationsdescribed above, a RAT for packed floating point data is provided withY*2 entries. Thus, for example, instead of a RAT with the entries for 8logical registers, a RAT is created with 16 entries, each addressed as“high” or “low.” Each entry identifies a 64-bit source corresponding toeither a high or a low part of the 128-bit logical register.

Each of the high and low order micro instructions thus has associatedentries in the register mapping table corresponding to the respectiveoperands. The micro instructions then move into a scheduling stage (foran out of order processor) or to an execution stage (for an in orderprocessor). Each micro instruction retrieves and separately processes a64-bit segment of the 128-bit operands. One of the operations (e.g. thelower order operation) is first executed by the 64-bit hardware units.Then, the same 64-bit hardware unit executes the higher order operation.It should be appreciated that zero or more instructions may be executedbetween the lower and higher order operations.

Although the above embodiment describes the macro instruction beingdivided into two micro instructions, alternate embodiments may dividethe macro instruction into more micro instruction. While FIG. 9 showsthat the packed floating point data is returned to a retirement registerfile with Y*2 64-bit registers, each designated as high or low,alternate embodiments may use a retirement register file with Y 128-bitregisters. In addition, while one embodiment is described having aregister renaming mechanism with a reorder buffer and retirementregister files, alternate embodiments may use any register renamingmechanism. For example, the register renaming mechanism of U.S. Pat. No.5,197,132 uses a history queue and backup map.

FIG. 10 is a timing chart that further illustrates the embodimentdescribed in FIGS. 9. At time T, a macro instruction “ADD X, Y” entersthe decode stage of the pipeline of FIG. 9. By way of example, the macroinstruction here is a 128-bit instruction. The 128-bit macro instructionis converted into two 64-bit micro instructions, namely the high orderoperation, “ADD X_(H), Y_(H)” and the low order operation, “ADD X_(L)Y_(L).” Each micro instruction then processes a segment of datacontaining two data elements. For example, at time T, the low orderoperation may be executed by a 64-bit execution unit. Then at adifferent time (e.g., time T+N), the high order operation is executed bythe same 64-bit execution unit. This embodiment of the invention is thusespecially suitable for processing 128-bit instructions using existing64-bit hardware systems without significant changes to the hardware. Theexisting systems are easily extended to include a new map to handlepacked floating point, in addition to the existing logical registermaps.

Referring now to FIG. 11, decoding logic that may be employed accordingto one embodiment of the invention is described. Briefly, in theembodiment depicted, a plurality of decoders 1110, 1120, and 1130 eachreceive a macro instruction and convert it into a micro instruction.Then the micro operating are sent down the remainder of the pipeline. Ofcourse, N micro instructions are not necessary for the execution ofevery macro instruction. Therefore, it is typically the case that only asubset of micro instructions are queued for processing by the remainderof the pipeline.

As described above, packed data operations may be implemented as twohalf width micro instructions (e.g., a high order operation and a loworder operation). Rather than independently decoding the macroinstruction by two decoders to produce the high and low order operationsas would be typically required by prior processor implementations, as afeature of the present embodiment both micro instructions may begenerated by the same decoder. In this example, this is accomplished byreplication logic 1150 which replicates either the high or low orderoperation and subsequently modifies the resulting replicated operationappropriately to create the remaining operation. Importantly, as wasdescribed earlier, by carefully encoding the register address space, theregisters referenced by the micro instructions (e.g., the logical sourceand destination registers) can be made to differ by a single bit. As aresult, the modification logic 1160 in its most simple form may compriseone or more inverters to invert the appropriate bits to produce a highorder operation from a low order operation and vice versa. In any event,the replicated micro instruction is then passed to multiplexer 1170. Themultiplexer 1170 also receives a micro instruction produced by decoder1120. In this example, the multiplexer 1170, under the control of avalidity decoder 1180, outputs the replicated micro instruction forpacked data operations (including partial-width packed data operations)and outputs the micro instruction received from decoder 1120 foroperations other than packed data operations. Therefore, it isadvantageous to optimize the opcode map to simplify the detection ofpacked data operations by the replication logic 1150. For example, ifonly a small portion of the of the macro instruction needs to beexamined to distinguish packed data operations from other operations,then less circuitry may be employed by the validity decoder 1180.

In an implementation that passes through source data elements to thelogical destination register for purposes of executing partial-widthpacked data operations, in addition to selection logic similar to thatdescribed with respect to FIGS. 5A and 5C, logic may be included toeliminate (“kill”) one of the high or low order operations. Preferably,for performance reasons, the extraneous micro instruction is eliminatedearly in the pipeline. This elimination may be accomplished according tothe embodiment depicted by using a micro instruction selection signaloutput from micro instruction length determination circuitry 1190. Themicro instruction length determination logic 1190 examines a portion ofthe macro instruction and produces the micro instruction selectionsignal which indicates a particular combination of one or more microinstructions that are to proceed down the pipeline. In the case of ascalar SIMD instruction, only one of the resulting high and low orderoperations will be allowed to proceed. For example, the microinstruction selection signal may be represented as a bit mask thatidentifies those of the micro instructions that are to be retained andthose that are to be eliminated. Alternatively, the micro instructionselection signal may simply indicate the number of micro instructionsfrom a predetermined starting point that are to be eliminated orretained. Logic required to perform the elimination described above willvary depending upon the steering mechanism that guides the microinstructions through the remainder of the pipeline. For instance, if themicro instructions are queued, logic would may be added to manipulatethe head and tail pointers of the micro instruction queue to causeinvalid micro instructions to be overwritten by subsequently generatedvalid micro instructions. Numerous other elimination techniques will beapparent to those of ordinary skill in the art.

Although for simplicity only a single macro instruction is shown asbeing decoded at a time in the embodiment depicted, in alternativeembodiments multiple macro instructions may be decoded concurrently.Also, it is appreciated that micro instruction replication has broaderapplicability than that illustrated by the above embodiment. Forexample, in a manner similar to that described above, full-width andpartial-width packed data macro instructions may be decoded by the samedecoder. If a prefix is used to distinguish full-width and partial widthpacked data macro instructions, the decoder may simply ignore the prefixand decode both types of instructions in the same manner. Then, theappropriate bits in the resulting micro operations may be modified toselectively enable processing for either all or a subset of the dataelements. In this manner, full-width packed data micro operations may begenerated from partial-width packed data micro operations or vice versa,thereby reducing complexity of the decoder.

Thus, a method and apparatus for efficiently executing partial-widthpacked data instructions are disclosed. These specific arrangements andmethods described herein are merely illustrative of the principles ofthe invention. Numerous modifications in form and detail may be made bythose of ordinary skill in the art without departing from the scope ofthe invention. Although this invention has been shown in relation to aparticular preferred embodiment, it should not be considered so limited.Rather, the invention is limited only by the scope of the appendedclaims.

What is claimed is:
 1. A processor comprising: a plurality of registers;a register renaming unit coupled to the plurality of registers toprovide an architectural register file to store packed data operands,each of said packed data operands having a plurality of data elements; adecoder, coupled to said register renaming unit, to decode a first andsecond set of instructions that each specify one or more registers inthe architectural register file, each instruction in the first set ofinstructions specifying operations on all of the data elements stored inthe specified one or more registers, each of the second set ofinstructions specifying an operation on only a subset of data elementstored in the specified one or more registers; and a partial-widthexecution unit, coupled to the decoder to execute operations specifiedby either of the first or the second set of instructions.
 2. Theprocessor of claim 1, wherein the subset of data elements stored in aspecified one or more registers comprises corresponding leastsignificant data elements.
 3. The processor of claim 1, furthercomprising an execution unit to selectively perform a specifiedoperation on one or more data elements in the specified one or moreregisters depending upon which of the first or second set ofinstructions the specified operation is associated.
 4. The processor ofclaim 3, wherein the execution unit further comprises a plurality ofmultiplexers to select between a result of the specified operation and apredetermined value.
 5. The processor of claim 3, wherein the executionunit further comprises a plurality of multiplexers to select between adata element of the one or more data elements and an identity functionfor input to the specified operation.
 6. The method of claim 1, whereinthe first subset of the corresponding data elements comprise a lowerorder data segment of the first and second packed data operands and thesecond subset of the corresponding data elements comprise a higher orderdata segment of the first and second packed data operands.
 7. A methodcomprising the steps of: receiving a single macro instruction specifyingat least two logical registers in a packed data register file, whereinthe two logical registers respectively store a first packed data operandand second packed data operand having corresponding data elements; andindependently operating on a first and second plurality of thecorresponding data elements from said first and second packed dataoperands at different times using the same circuit to independentlygenerate a first and second plurality of resulting data elements byperforming an operation specified by the single macro instruction on atleast one pair of corresponding data elements in the first and secondplurality corresponding data elements to produce at least one resultingdata element of the first and second plurality of resulting dataelements, and setting remaining resulting data elements of the first andsecond plurality of resulting data elements to one or more predeterminedvalues; and storing the first and second plurality of resulting dataelements in a single logical register as a third packed data operand. 8.The method of claim 7, wherein the one or more predetermined valuescomprise values of data elements from either the first packed dataoperand or the second packed data operand.
 9. The method of claim 7,wherein the one or more predetermined values comprise zero.
 10. Themethod of claim 7, wherein the one or more predetermined values comprisea not-a-number (NaN) indication.
 11. The method of claim 7, wherein thesingle macro instruction is a packed data macro instruction.
 12. Aprocessor comprising: a plurality of registers; a register renaming unitcoupled to the plurality of registers to provide an architecturalregister file to store packed data operands, each of said packed dataoperands having a plurality of data elements; a decoder, coupled to saidregister renaming unit, to decode a first and second set of packed datainstructions that each specify one or more registers in thearchitectural register file, each packed data instruction in the firstset of packed data instructions specifying operations on all of the dataelements of one or more packed data operands stored in the specified oneor more registers, each of the second set of packed data instructionsspecifying an operation on only a subset of data elements of one or morepacked data operands stored in the specified one or more registers; anda partial-width execution unit, coupled to the decoder to executeoperations specified by either of the first or the second set of packeddata instructions.
 13. The processor of claim 12, wherein the subset ofdata elements stored in a specified one or more registers comprisescorresponding least significant data elements.
 14. The processor ofclaim 12, further comprising an execution unit to selectively perform aspecified operation on one or more data elements in the specified one ormore registers depending upon which of the first or second set ofinstructions the specified operation is associated.
 15. The processor ofclaim 14, wherein the execution unit further comprises a plurality ofmultiplexers to select between a result of the specified operation and apredetermined value.
 16. The processor of claim 14, wherein theexecution unit further comprises a plurality of multiplexers to selectbetween a data element of the one or more data elements and an identityfunction for input to the specified operation.
 17. A method comprising:receiving a single full-width macro instruction specifying at least twological registers, the two logical registers respectively storing firstand second packed data operands having corresponding data elements;responsive to the single full-width macro instruction, dividing thesingle full-width macro instruction into at least two partial-widthmicro instructions that each correspond to a subset of the correspondingdata elements of the first and second packed data operands, bygenerating a first and second partial-width micro instructioncorresponding to the single full-width macro instruction; eliminating anoperation on a second subset of the corresponding data elements fromsaid first and second packed data operands; accessing a first subset ofthe corresponding data elements from the first and second logicalregisters in response to the first partial-width micro instruction; andresponsive to the first partial-width micro instruction, performing anoperation specified by the single full-width macro instruction on thefirst subset of the corresponding data elements from said first andsecond packed data operands, to generate a first subset of resultingdata elements, the first subset of resulting data elements being storedin a portion of a single logical register as a third packed dataoperand.
 18. A method as in claim 17, further comprising setting theremainder of the single logical register to one or more predeterminedvalues.
 19. A method as in claim 18, additionally comprising eliminatingthe second partial-width micro instruction.
 20. A method as in claim 18,wherein the one or more predetermined values comprises values from dataelements in one of the first and second packed data operands from thesecond subset of corresponding data elements.
 21. A method as in claim18, wherein the predetermined values comprises zero.
 22. A method as inclaim 18, wherein the predetermined values comprises a not-a-number(NaN) indicator.
 23. A method comprising: receiving a single macroinstruction specifying an operation; generating a first microinstruction and second micro instruction corresponding to the singlemacro instruction, the first micro instruction specifying a firstoperation and the second micro instruction specifying a secondoperation; a partial-width execution unit executing the first operationspecified by the first micro instruction on a first subset of aplurality of data elements to generate a first set of resulting dataelements to be stored in a portion of a single logical register; andeliminating the second operation specified by the second microinstruction.
 24. A method as in claim 23, further comprising setting theremainder of the single logical register to one or more predeterminedvalues.
 25. A method as in claim 24, additionally comprising eliminatingthe second micro instruction.
 26. A method as in claim 24, wherein thepredetermined values comprises zero.
 27. A method as in claim 24,wherein the predetermined values comprises a not-a-number (NaN)indicator.
 28. A processor comprising: a register file containing packeddata wherein the data includes a plurality of data elements; apartial-width execution unit coupled to the register file; and acircuit, coupled to the register file and the execution unit, inresponse to a single full-width macro instruction specifying anoperation, being configured to: divide the single full-width macroinstruction into at least two partial-width micro instructions, thateach correspond to a subset of corresponding data elements of theplurality of data elements, by generating a first and secondpartial-width micro instruction corresponding to the single full-widthmacro instruction; cause the partial-width execution unit, responsive tothe first partial-width micro instruction, to perform the operation on afirst subset of the plurality of data elements to output a first resultthat is stored in a portion of a single logical register; and cause thepartial-width execution unit to eliminate performing the operation onthe second subset of the plurality of data elements.
 29. The processorof claim 28, wherein the plurality of data elements includes 128-bits ofdata from the register file.
 30. The processor of claim 29, wherein thefirst and second subset of the plurality of data elements each include64-bits of data from the register file.
 31. A processor of claim 28,further comprising setting the remainder of the single logical registerto one or more predetermined values.
 32. A machine-readable mediumhaving stored thereon data representing sequences of instructions, thesequences of instructions which, when executed by a processor, cause theprocessor to perform the operations of: receiving a single full-widthmacro instruction specifying at least two logical registers, wherein thetwo logical registers respectively store a first and second packed dataoperands having corresponding data elements; responsive to the singlefull-width macro instruction, dividing the single full-width macroinstruction into at least two partial-width micro instructions, thateach correspond to a subset of the corresponding data elements of thefirst and second packed data operands, by generating a first and secondpartial-width micro instruction corresponding to the single full-widthmacro instruction; eliminating an operation on a second subset of thecorresponding data elements from said first and second packed dataoperands, independently accessing a first subset of the correspondingdata elements from the first logical register in response to the firstpartial-width micro instruction; and responsive to the firstpartial-width micro instruction, performing an operation specified bythe single full-width macro instruction on the first subset of thecorresponding data elements from said first and second packed dataoperands to independently generate a first plurality of resulting dataelements, wherein the first plurality of resulting data elements isstored in a single logical register as a third packed data operand. 33.The machine-readable medium of claim 32, wherein the first subset of thecorresponding data elements comprise a lower order data segment of thefirst and second packed data operands and the second subset of thecorresponding data elements comprise a higher order data segment of thefirst and second packed data operands.
 34. The machine-readable mediumof claim 32, further comprising setting the remainder of the singlelogical register to one or more predetermined values.